I shall highlight the more important ones here:
Shares:
1) normal brokerage charges applies
2) transaction fee between $2 and $2.50 per 1000 units/shares (up to max of $20-$25)
3) service charge of $2 per counter per quarter.

Based on these charges, I will suggest
1) DO NOT buy penny stocks using your CPF-OA. The one time transaction fee will eat into your overall costs
2) try not to over diversify on your CPF-OA stocks. Each counter will eat up $2 per quarter.
3) calculate your overall return carefully, taking all these charges into consideration. You need to beat the 2.5% to make it worthwhile to invest your CPF-OA